1. Field of the Invention
This invention relates to conveyor chain and in particular to long courses of conveyor chain in systems that require lateral turns in the conveying path.
2. Description of the Prior Art
U.S. Pat. No. 3,282,121 is known to apply to the subject matter of this invention. It discloses a drive chain capable of driving a plurality of sprockets lying on diverse planes, and accomplishes this result through the use of flexible links capable of twisting about the longitudinal axis of the chain. Most chain has some capacity to twist, largely the result of manufacturing tolerances, which combine to produce clearances at the joints. When the clearances are taken up by rotation of mating parts relative to one another at each joint, the chain appears to twist. The apparent twisting results even though no link of the chain actually experiences torsion.
Conveyor chain has conventionally been fabricated from metal. Cost influences require that machining be maintained at minimum amounts; therefore, the chain is assembled as a composite of elements, each of which can be produced with minimum cost processes. This approach in metal, however, has the disadvantage of requiring a high parts count and greater weight than if plastic links were used. Other disadvantages are inherent in metal fabrication particularly where marring of the conveyed article is to be avoided. The use of plastic for certain chain components of this type can reduce the number of parts required, lighten the chain weight, reduce machining and avoid the marring problem. In addition, certain plastics are readily available in the commercial market, which, because of their inherent lubricious nature, can eliminate the need to lubricate chain, a particularly attractive advantage in certain markets where the conveyed article is sensitive to contamination, as in the food industry. The invention contemplates the disadvantages with the existing art and uniquely exploits the lubricious characteristics of plastic to provide a block link that contacts the sprocket tooth faces, and thereby eliminates the usual requirement for rolling contact between tooth and link.
When lateral turns are required in a conveyor system that has links formed of plastic, the standard metal chain link design must be modified to avoid large bearing stresses and wear points that arise at joints as they rotate relative to one another while in the turn. Plastic has an allowable bearing strength considerably less than conventional chain metal allowables. Unless suitably provided for, turns, which produce apparent chain bending, will develop bearing stresses among mating parts far in excess of plastic material allowables. Flexible link length is chosen upon taking due account of the material stiffness, thickness, width and end restraint conditions that are required to establish the edgewise bending flexibility compatible with the radius of curvature of the turns to be negotiated. The intent is to provide sufficient link bending flexibility so as to limit joint rotation, and thereby to minimize peak bearing stresses, that develop between mating parts at the joints.
In addition to overcoming the bearing stress problem where plastic links are used, the disclosed invention makes possible the attainment of other desirable plastic chain features while avoiding troublesome aspects of its use.